Metallocene catalysts (Kaminsky catalysts) used together with methylaluminoxane (MAO) as a co-catalyst have been found to be useful as catalysts for the polymerization of olefins, and have been extensively studied. However, in order to allow the metallocene catalysts to exhibit its catalytic activity, it is necessary to use therewith a large amount of the expensive methyl aluminoxane. To solve the problem, for example, Japanese Patent Application Laid-Open No. 5-25214 has proposed to use a methylalumoxane co-catalyst supported on clay minerals (one of silicon-containing layered compounds) as a catalyst component for olefin-polymerizing catalyst, and specifically describes a methylalumoxane co-catalyst supported on smectite (clay minerals are used as a catalyst support). However, the preparation of such a co-catalyst inevitably requires to treat clays with a large amount of expensive and harmful methylalumoxane, and the obtained catalyst is still insufficient in polymerization activity per unit quantity of the aluminum component used. In addition, U.S. Pat. No. 5,308,811, etc., have proposed the use of clays treated with an aluminum compound such as trialkylaluminiun as a co-catalyst, and specifically describes a catalyst composition comprising a metallocene complex, clay minerals, etc. However, the catalyst composition fails to achieve a sufficient polymerization activity for vinyl compounds only by treating clays with ordinary organoaluminum compounds. WO 99/02472 describes a process for producing α-olefins using a catalyst composed of a specific iron complex and an organoaluminum compound, and also describes the use of acidic clays such as montmorillonite as a support of activated catalysts or catalyst precursors. However, when methylaluminoxane described as a specific example of the organoaluminum compound is used as a catalyst component, the catalytic activity, especially the activity per unit quantity of aluminum, is still unsatisfactory.
Japanese Patent Application Laid-Open No. 8-295705 describes a process for producing α-olefins by the oligomerization of ethylene, in which α-olefins are separated from a reaction product to recover a catalyst and by-produced polymers. In this process, after separating α-olefins by distillation using an evaporator, the by-produced polymer and catalyst are recovered in the concentrated liquid residue. Also, Japanese Patent Application Laid-Open No. 10-45833 describes a process in which a reaction product solution is kept at a high temperature in order to inhibit the precipitation of by-produced polymer and facilitate subsequent treatments. In these conventional processes, since α-olefins are separated by distillation in the presence of the catalyst components, α-olefins are susceptible to side reactions such as isomerization. Further, Japanese Patent Application Laid-Open No. 7-149671 describes a process in which after preliminarily separating by-produced polymer, an ethylene oligomer (α-olefin) is separated and purified. In any of the prior art processes, ethylene is trimerized in the presence of a chromium catalyst to produce oligomers thereof. Further, these processes require an additional step for subjecting the catalyst components to deactivation or deashing treatment for removal thereof. In particular, in the process described in Japanese Patent Application Laid-Open No. 7-149671, since a homogeneous catalyst system comprising a chromium complex is used to enhance the catalytic activity, it is not possible to completely remove the catalyst components upon removal of the by-produced polymer. In addition, a pyrrole compound added to improve the activity of the chromium catalyst is difficult to remove, resulting in failure to purify α-olefins having 8 or more carbon atoms.